Understanding Common Furnace Thermostats

Standard Furnace Thermostats

The furnace thermostat is the nerve center of your home heating system. It's the main control point that determines when and how much heat will be delivered by the furnace. But for all its mystery, the thermostat is actually a very simple device. At its most basic level, it's simply a temperature-sensitive switch.

Most thermostats that control central heating equipment, including furnaces, boilers, and heat pumps, are low-voltage thermostats that typically use 24 volts of electricity. Electric baseboard heaters and in-wall heaters may use line-voltage thermostats that use 120 or 240 volts of electricity.

Low-voltage thermostats are powered by a transformer that reduces the incoming line voltage from 120 volts down to a level between 12 and 24 volts, depending on your furnace design. Within this low-voltage thermostat category, there are four major types:

Mercury Contact Bimetallic Thermostat

The mercury contact bimetallic thermostat is an older type of analog (non-digital) thermostat and once was the most common type found in homes. Its simplicity makes it a good example for describing how an analog thermostat works.

For an analog thermostat to work, something has to move as the temperature increases or decreases. That's the job of the bimetallic strip. "Bimetallic" is just a fancy name for two pieces of metal joined together. But the reason it works is that the two metals expand at different rates at a given temperature (known as the coefficient of expansion). As one side expands more than the other side, the metal curves one way or the other. You then couple this little bit of physics knowledge with the exact rates of expansion for the two metals, and the result is a precise instrument that will move a specific amount at a given temperature.

The thermostat contact is like a switch for your lights. As the bimetallic strip moves, it either opens or closes the contact. In the case of a mercury thermostat, the contact is a glass vial, called an ampoule, containing mercury. The ampoule is attached to a coiled bimetallic strip, also called a bimetallic coil. As the coil moves, it tips the vial of mercury until the mercury moves and the switch contact is opened or closed.

Finally, the heat anticipator is an electrical resistance wire mounted on a center disc connected to the bimetallic coil. It is adjustable to allow fine-tuning of when the thermostat turns the furnace blower on and off. The bimetallic thermostat has modes for heating and cooling and fan positions for on/off and automatic. It is not programmable.

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Mechanical Contact Thermostat

The mechanical contact thermostat uses a simple mechanical contact instead of a mercury contact switch. Like a mercury contact thermostat, a mechanical thermostat uses a bimetallic coil (to open or close the mechanical contact points) and includes a ​heat anticipator. Some older versions of mechanical contact thermostats used a straight blade bimetallic strip and not a coiled strip. They are a bit less accurate but work under the same principle.

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Digital Thermostat

The ​digital thermostat is an electronic version of the simple mechanical thermostat. It uses electronic controls instead of a bimetallic coil to sense temperature changes and is much more accurate. These thermostats usually have large, easy-to-read displays and are often used to replace older mechanical thermostats. Digital thermostats have modes for heating and cooling and fan positions for on/off and automatic, but they are not programmable.

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Electronic Programmable Thermostat

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The electronic programmable thermostat takes the digital thermostat and gives it a "brain." It has user controls that allow you to set the days of the week, the time, and the temperature as well as ​custom programming programming functions. An electronic programmable thermostat can save energy by allowing you to set back your heat at night or when you are away. You can also set different programs for weekdays and weekends, and some can even tell you when you need to change your ​furnace filter.

The latest iteration of the programmable thermostat is the smart thermostat, such as the Nest. These are fully programmable and can be controlled manually or with a smartphone or other device. They are also capable of programming themselves by learning how you typically set the temperature throughout the week.